Among non-aqueous electrolyte secondary batteries, lithium ion secondary batteries using a material capable of absorbing and desorbing lithium ions represented by carbon materials as a negative electrode active material have been in practical use. For the negative electrode of such lithium ion secondary batteries, generally, graphite and carbon black are used as conductive agents.
In a negative electrode including such a negative electrode active material and a conductive agent, the charge and discharge capacity per unit mass has been improved to nearly reach the theoretical capacity, and therefore the energy density per unit mass is reaching its limit.
Thus, to improve the electrode utilization rate, there has been an attempt to decrease the amount of the material not contributing to the discharge capacity in the electrode (for example, binders and conductive agents).
For example, Japanese Laid-Open Patent Publication No. 2005-222933 (Document 1) has proposed a negative electrode using styrene butadiene rubber (abbreviated as SBR) as the binder and plural kinds of vapor-grown carbon fibers as the conductive agent. Such a negative electrode is excellent in cycle characteristics and high-speed charge and discharge characteristics, as well as high in energy density. The SBR mentioned above functions with a small amount, compared with fluorocarbon resins represented by polyvinylidene fluoride conventionally used as the negative electrode binder. In addition, with SBR, manufacturing steps can be simplified, since it is used as an aqueous dispersion. The vapor-grown carbon fiber is highly electrically conductive compared with conventional carbon black such as acetylene black, and also is able to improve electrode strength.
Further, with the trend toward small size, lightweight, and high performance mobile devices, lithium ion secondary batteries are also required to have high capacity. Silicon (theoretical capacity: 4199 mAh/g) type active materials have been proposed in place of carbon material type active materials represented by graphite (theoretical capacity: 372 mAh/g). However, the silicon type active materials undergo a high degree of volume change when lithium ions are absorbed and desorbed. For example, when silicon simple substance maximally absorbs lithium upon charging, the volume of silicon simple substance theoretically increases up to 4.1 times the volume of silicon simple substance not absorbing lithium. On the other hand, in the case of graphite, since the intercalation reaction is used, lithium is intercalated between the graphite layers. Thus, the volume of graphite with lithium absorbed increases only up to about 1.1 times the volume of graphite not absorbing lithium.
Thus, in the case of the silicon type active materials, gaps are created between the active material particles due to its high degree of volume change, and therefore the negative electrode portion that effectively contributes to the battery capacity decreases. Further, the volume change causes cracks in the active material particles, making the active material particles finer. When the active material particles are made finer, the electron conduction network based on contacts between the active material particles is disconnected. Thus, the negative electrode portion that cannot contribute to the electrochemical reaction (the portion that cannot contribute to the battery capacity) increases. This leads to a decrease in charge and discharge capacity, or an increase in internal resistance.
Japanese Laid-Open Patent Publication No. 2004-178922 (Document 2) has proposed mixing particles containing a compound including silicon atoms with vapor-grown carbon fiber, and covering at least a portion of the surface of the particles containing the compound including silicon atoms with a carbonaceous material such as phenolic resin.
The negative electrode including a current collector and a material mixture layer containing the active material carried thereon is made by mixing an electrode material mixture with water or an organic solvent to obtain a material mixture paste, and applying the paste to the current collector. In such a negative electrode, the current collector and the material mixture layer are bonded by a binder. The change of the current collector size upon charge and discharge is small, and the thickness of the material mixture layer carried on the current collector is thinner than the thickness of a pellet of a molded body made of a negative electrode material mixture. Thus, the bond between the current collector and the negative electrode material mixture can be kept easily. Further, in the material mixture including vapor-grown carbon fiber and the particles containing a compound including silicon atoms, contacts between the active material particles and the conductive agent can be further easily kept compared with the material mixture using carbon black as the conductive agent, due to the use of fibrous conductive agent.
Further, in the case of the active material particles in which at least a portion of the surface of the particles containing a compound including silicon atoms is covered with a carbonaceous material, charge and discharge cycle characteristics and low temperature characteristics can be improved to a certain degree.
However, in the case of the negative electrode not including a current collector and formed only of a pellet of a bulky molded body, its thickness is more than the thickness of the electrode including the current collector and the active material layer. Thus, with repetitive expansion and contraction of the active material upon charge and discharge, compared with the electrode including the current collector and the active material layer, the degree of expansion and contraction is high. Therefore, in a negative electrode made with the molded body, the electric conductivity between the active material particles cannot be kept just by mixing the active material particles and the vapor-grown carbon fiber, and charge and discharge cycle characteristics drastically declines.
Conventionally, the decline in charge and discharge cycle characteristics in batteries including the carbon material type active material has been regarded as due to the decline in current collecting ability. On the other hand, in batteries including the negative electrode active material containing Si, the decline in charge and discharge cycle characteristics is probably due not only to the decline in current collecting ability, but also to the fact that with the repetitive expansion and contraction of the active material including Si, a new surface is created on the active material, and a film derived from the non-aqueous electrolyte is formed on the surface thereof. Thus, to curb the decline in charge and discharge cycle characteristics, it is important to curb the formation of the film, and to keep the electric conductivity between the active material particles.